1. Field of the Invention
The present invention relates to electronic ignition systems and more particularly to an electronic dwell time control system for an internal combustion engine wherein the time of starting the flow of primary current is determined by the output signal of a primary coil control circuit which is computed in accordance with the charging and discharging of at least one capacitor.
2. Description of the Prior Art
In a known type of ignition system in which current is supplied to the primary winding of the ignition coil and then the flow of this primary current is interrupted to induce a high voltage in the secondary winding, the point at which the flow of the primary current is started must be properly determined in addition to the ignition timing or the point at which the flow of primary current is interrupted. And it is desirable that the length of time during which the primary current flows in the ignition coil is maintained constant in consideration of the requirements of the ignition coil and the ignition energy. In a known type of mechanical ignition timing control system, the timing of opening and closing the contact points which connect and disconnect the primary winding with the ground is shifted in relation to the top dead center by the centrifugal governor and the angle during which the contact points are closed is always fixed irrespective of the rotational speed of the engine. On the other hand, in a known type of ignition system in which the ignition timing is electronically determined, the point at which the flow of primary current is started must be separately determined.
While these conventional ignition timing control systems have many advantages, they also have some disadvantages. In other words, while the above-described mechanical contact system is advantageous in that the angle during which the primary current flows is always maintained constant, there is a limit to the degree of freedom in the control of ignition timing, whereas in the case of the electronically controlled ignition timing it is necessary to separately compute the proper time instant for starting the flow of primary current, although there is the advantage of an increased degree of freedom in the control of ignition timing.
Moreover, with the regulations on the exhaust emission becoming increasingly severe in these days, optimum control of the ignition timing of an internal combustion engine in response to the engine operating conditions has become a very important problem in view of the desired reduction in exhaust emissions and improvement in the power output and fuel consumption. On the other hand, there has been a demand for exhaust emission control systems which are simple and inexpensive.